An X-ray CT apparatus provides information on a subject as an image on the basis of the strengths of the X-rays that have passed through the subject. The X-ray CT apparatuses play an important role in various medical practices such as diagnosis and treatment of a disease and operative planning.
A CT image, which is used as a diagnostic image, obtained by an X-ray CT apparatus may, in some cases, include a group of pixels having a high CT value (a high CT value area) with halation, such as a calcified or a stent (metal) area. A CT image including a high CT area cannot provide an accurate evaluation of a lumen of a blood vessel. As an approach to accurately evaluate a lumen of a blood vessel, there is a method for generating a subtraction image as a diagnostic image by performing subtraction processing in order to delete a high CT value area from a diagnostic image. A simple CT image including a high CT value area and a contrast CT image including a high CT value area obtained by an X-ray CT apparatus are used to perform subtraction processing between both the images.
However, according to the conventional technique, burring may be found in a high CT value area occurring in each of a simple CT image and a contrast CT image due to positional deviations caused by cardiac movements and a cardiac phase difference (respiratory movements). In addition, generally, since a simple CT image has no reference information that can be used for alignment (i.e. much less CT value distribution representative of a characteristic feature is available than a contrast CT image), a method that uses normalized mutual information (NMI) of a simple CT image and a contrast CT image as similarity and a non-linear alignment based on anatomical information are both inaccurate.
Unfortunately, subtraction processing between a simple CT image and a contrast CT image in accordance with the conventional technique may leave a false image of a high CT value area caused by blurring on a subtraction image.